DLS：无目录共享末级缓存
DLS: Directoryless Shared Last-level Cache
摘要：
基于目录的协议已经成为用于在包括多/许多核的共享存储器并行系统中维持缓存一致性的事实上的解决方案，其中通过使其他核的私有缓存（PC）备份无效来急切地使每个存储指令全局可见。因此，目录不仅消耗大的芯片面积，而且由于在互连网络中传输的大量无效/Ack消息和导致的网络拥塞而引起相当大的能量消耗和性能下降。在本文中，我们揭示了一个有趣的事实，目录实际上是一个不必要的奢侈品，实际的并行系统。由于广泛部署的涉及指令重新排序的软件/硬件技术，大多数（如果不是全部的话）并行系统在弱一致性模型下工作，其中允许远程存储指令在核的下一次同步之前对于核是不可见的，而不是通过使其它核的PC备份无效而急切地变得可见。基于这一关键的观察，我们提出了一个轻量级的新方案，称为{\em DLS（DirectoryLess Shared last-level cache）}，它完全删除了目录和无效/Ack消息，并有效地保持缓存一致性使用一种新的{\em自我怀疑+推测执行}机制。SPLASH-2基准测试结果表明，在16核处理器上，DLS不仅完全消除了目录的芯片面积开销，而且处理器性能提高了11.08%，整体网络流量降低了28.83%，网络能耗平均降低了15.65%（与传统的全目录MESI协议相比）.此外，DLS不涉及对程序设计语言和编译器的任何修改，因此与遗留代码无缝兼容。
英文摘要：
Directory-based protocols have been the de facto solution for maintaining cache coherence in shared-memory parallel systems comprising multi/many cores, where each store instruction is eagerly made globally visible by invalidating the private cache (PC) backups of other cores. Consequently, the directory not only consumes large chip area, but also incurs considerable energy consumption and performance degradation, due to the large number of Invalidation/Ack messages transferred in the interconnection network and resulting network congestion. In this paper, we reveal the interesting fact that the directory is actually an unnecessary luxury for practical parallel systems. Because of widely deployed software/hardware techniques involving instruction reordering, most (if not all) parallel systems work under the weak consistency model, where a remote store instruction is allowed to be invisible to a core before the next synchronization of the core, instead of being made visible eagerly by invalidating PC backups of other cores. Based on this key observation, we propose a lightweight novel scheme called {\em DLS (DirectoryLess Shared last-level cache)}, which completely removes the directory and Invalidation/Ack messages, and efficiently maintains cache coherence using a novel {\em self-suspicion + speculative execution} mechanism. Experimental results over SPLASH-2 benchmarks show that on a 16-core processor, DLS not only completely removes the chip area cost of the directory, but also improves processor performance by 11.08%, reduces overall network traffic by 28.83%, and reduces energy consumption of the network by 15.65% on average (compared with traditional MESI protocol with full directory). Moreover, DLS does not involve any modification to programming languages and compilers, and hence is seamlessly compatible with legacy codes.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
